Biology 12

MINI LAB

What Is the Advantage

of Binocular Vision?

In this lab you will work with a partner to briefly explore how
binocular vision is important. You will need a long pencil and
a 10 mL graduated cylinder. Hold the end of the pencil
between two fingers. Have your partner stand about 2 m
away, holding the graduated cylinder with his or her arm
extended toward you. Cover one eye and then, in one
motion, attempt to insert the point of the pencil into the
opening at the top of the graduated cylinder. Try this with
the other eye covered and then with both eyes open.
Repeat the experiment, switching roles. Be sure that the
student with the pencil uses one motion in attempting to
insert the pencil into the graduated cylinder. Doing this

slowly gives the brain additional information to allow it to

judge the distance.

Analyze

1.Did using both eyes at the same time help in judging

distances?

2.Was this task easier using one eye (right or left) than

the other?

3.Most people have a dominant eye that is favoured

by the brain in gathering and interpreting information.

Design a method of determining which is your

dominant eye.

4.Explain how binocular vision works using the feedback

model from Chapter 4 (receptor, integrator, and effector).

Chapter 5 The Nervous System • MHR 153

retina. The sclerais the thick, white, outer layer
that gives the eye its shape. At the front of the eye
the sclera bulges outward and becomes clear to
form the cornea. The cornea is covered by a thin,
transparent membrane — the conjunctiva— that
is continually kept moist by fluid from the tear
glands. The fluid is drained off through small
ducts that lead to the nasal cavities.

The middle layer is the coloured choroid layer,

which absorbs light and prevents internal reflection.

The light absorbed by the choroid layer is that

which has not been absorbed by the sclera. At the

front of the eye, the choroid becomes the iris, which

opens and closes to control the size of the pupil.

Behind the iris, the choroid thickens and forms

the ciliary body, which contains muscles to control

the shape of the lens. The clear, flexible lens

changes as you look at objects close up or far away.

The lens is attached to suspensory ligamentsthat

are, in turn, attached to the ciliary muscles.

The inner layer of the eye, the retina, is

composed of two types of photoreceptors — rods

and cones. Rods are more sensitive to light than

Lysozyme, an enzyme that destroys bacteria by dissolving

their cell membranes, was discovered in the fluid from the

tear glands by Canadian bacteriologist Alexander Fleming in

1922. Fleming also discovered penicillin in 1925.

BIO FACT

ciliary body

lens

iris

pupil

cornea

anterior compartment

filled with aqueous humour

posterior compartment

filled with vitreous humour

retina

choroid

sclera

retinal blood vessels

optic nerve

fovea centralis

Figure 5.15Notice that the sclera, the outer layer of the
eye, becomes the cornea and that the choroid, the middle
layer, is continuous with the ciliary body and the iris.

The retina, the inner layer of the eye, contains the

photoreceptors for vision; the fovea centralis is the

region where vision is most acute.